NMR imaging or spectroscopy involves the production of a relatively strong, steady, and spatially uniform magnetic field in an object to be examined, and perturbing radio-frequency (RF) signals. When the frequency of the RF signals is related to the strength of the magnetic field by the gyro-magnetic ratio of nuclei of interest in the object, said nuclei are excited, or perturbed, by the RF signals. Upon termination of the RF signals, the excited nuclei relax thereby emitting what are commonly referred to as NMR signals at the same frequency as the perturbing RF signals. The NMR signals are received by a similar, or the same, antenna coil.
Each circuit path in a living object, by reason of its temperature and resistivity, constituted a small noise source which is coupled to the receiving antenna coil. Consequently, the received signal includes a component from these noise sources which reduces the signal-to-noise ratio of the system.
While it is possible to measure the NMR signal for a local sub-volume of an object (e.g., the spine of a living patient), the contribution of noise from all of the object volume severely compromises the signal-to-noise ratio and reduces image quality. An additional problem in localized imaging is the non-discriminate delivery of RF power into the entire volume of the patient during excitation. This is costly to generate, and results in unnecessary heat dose to non-imaged regions of the patient.
A conventional way to improve the signal-to-noise ratio in both NMR spectroscopy and imaging is to localize the transmitted and received fields using a surface coil antenna that basically is a planar loop of wire positioned adjacent the region of the patient to be imaged. Such loop will discriminately excite and receive signals from only a limited sub-region of a patient. This approach has many shortcomings: multiple side lobes, highlighted near-skin signals with reduced signals from the actual region of interest when such region is interior to the patient, capacitive effects with the patient, and sensitivity of the results to patient movement. In addition, a surface coil, by its nature, is capable of receiving radiation from only a limited solid angle with the result that much of the radiated signal emitted by an excited nucleus is lost thus limiting the actual signal-to-noise ratio.
It is therefore an object of the present invention to provide a new and improved NMR antenna system which does not suffer from the deficiencies of the prior art, and which has an improved signal-to-noise ratio as compared to the prior art, and to a method for designing such improved NMR antenna system.